Poach the shrimp gently: drop them into well-salted simmering water, kill the heat, cover, and let them sit 3 to 5 minutes until just opaque and curled into a loose C.
Move the shrimp straight into ice water for 5 minutes, then drain. Gentle heat plus a hard chill keeps them tender.
Stir the tomato juice, ketchup, lime juice, hot sauce and salt into a cocktail base in a large bowl.
Fold in the cucumber, onion, tomato and jalapeno.
Add the chilled shrimp, halving any giants, and refrigerate at least 30 minutes so the flavors marry.
Fold in the avocado and cilantro just before serving. Serve cold in glasses with saltines or tostadas and extra lime.